NAME

       pthread_setaffinity_np, pthread_getaffinity_np - set/get CPU affinity of a thread

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <pthread.h>

       int pthread_setaffinity_np(pthread_t thread, size_t cpusetsize,
                                  const cpu_set_t *cpuset);
       int pthread_getaffinity_np(pthread_t thread, size_t cpusetsize,
                                  cpu_set_t *cpuset);

       Compile and link with -pthread.

DESCRIPTION

       The  pthread_setaffinity_np()  function  sets  the  CPU affinity mask of the thread thread to the CPU set
       pointed to by cpuset.  If the call is successful, and the thread is not currently running on one  of  the
       CPUs in cpuset, then it is migrated to one of those CPUs.

       The  pthread_getaffinity_np()  function  returns the CPU affinity mask of the thread thread in the buffer
       pointed to by cpuset.

       For more details on CPU affinity masks, see sched_setaffinity(2).  For a description of a set  of  macros
       that can be used to manipulate and inspect CPU sets, see CPU_SET(3).

       The  argument  cpusetsize  is  the length (in bytes) of the buffer pointed to by cpuset.  Typically, this
       argument would be specified as sizeof(cpu_set_t).  (It may be some  other  value,  if  using  the  macros
       described in CPU_SET(3) for dynamically allocating a CPU set.)

RETURN VALUE

       On success, these functions return 0; on error, they return a nonzero error number.

ERRORS

       EFAULT A supplied memory address was invalid.

       EINVAL (pthread_setaffinity_np())  The  affinity  bit mask mask contains no processors that are currently
              physically on the system and permitted to the thread according to any  restrictions  that  may  be
              imposed by the "cpuset" mechanism described in cpuset(7).

       EINVAL (pthread_setaffinity_np())  cpuset  specified  a  CPU  that  was  outside the set supported by the
              kernel.  (The kernel configuration option CONFIG_NR_CPUS defines the range of the set supported by
              the kernel data type used to represent CPU sets.)

       EINVAL (pthread_getaffinity_np())  cpusetsize  is  smaller than the size of the affinity mask used by the
              kernel.

       ESRCH  No thread with the ID thread could be found.

VERSIONS

       These functions are provided by glibc since version 2.3.4.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────┬───────────────┬─────────┐
       │Interface                 │ Attribute     │ Value   │
       ├──────────────────────────┼───────────────┼─────────┤
       │pthread_setaffinity_np(), │ Thread safety │ MT-Safe │
       │pthread_getaffinity_np()  │               │         │
       └──────────────────────────┴───────────────┴─────────┘

CONFORMING TO

       These functions are nonstandard GNU extensions; hence the suffix "_np" (nonportable) in the names.

NOTES

       After  a  call  to pthread_setaffinity_np(), the set of CPUs on which the thread will actually run is the
       intersection of the set specified in the cpuset argument and the set of  CPUs  actually  present  on  the
       system.   The  system  may  further  restrict  the  set  of CPUs on which the thread runs if the "cpuset"
       mechanism described in cpuset(7) is being used.  These restrictions on the actual set of  CPUs  on  which
       the thread will run are silently imposed by the kernel.

       These functions are implemented on top of the sched_setaffinity(2) and sched_getaffinity(2) system calls.

       In  glibc  2.3.3 only, versions of these functions were provided that did not have a cpusetsize argument.
       Instead the CPU set size given to the underlying system calls was always sizeof(cpu_set_t).

       A new thread created by pthread_create(3) inherits a copy of its creator's CPU affinity mask.

EXAMPLE

       In the following program, the main thread uses pthread_setaffinity_np() to set its CPU affinity  mask  to
       include   CPUs   0   to   7   (which   may   not  all  be  available  on  the  system),  and  then  calls
       pthread_getaffinity_np() to check the resulting CPU affinity mask of the thread.

       #define _GNU_SOURCE
       #include <pthread.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <errno.h>

       #define handle_error_en(en, msg) \
               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

       int
       main(int argc, char *argv[])
       {
           int s, j;
           cpu_set_t cpuset;
           pthread_t thread;

           thread = pthread_self();

           /* Set affinity mask to include CPUs 0 to 7 */

           CPU_ZERO(&cpuset);
           for (j = 0; j < 8; j++)
               CPU_SET(j, &cpuset);

           s = pthread_setaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
           if (s != 0)
               handle_error_en(s, "pthread_setaffinity_np");

           /* Check the actual affinity mask assigned to the thread */

           s = pthread_getaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
           if (s != 0)
               handle_error_en(s, "pthread_getaffinity_np");

           printf("Set returned by pthread_getaffinity_np() contained:\n");
           for (j = 0; j < CPU_SETSIZE; j++)
               if (CPU_ISSET(j, &cpuset))
                   printf("    CPU %d\n", j);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       sched_setaffinity(2),  pthread_attr_setaffinity_np(3),   pthread_self(3),   sched_getcpu(3),   cpuset(7),
       pthreads(7), sched(7)

COLOPHON

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       http://www.kernel.org/doc/man-pages/.